Manufacturers of integrated circuits often need to pattern polysilicon structures over a non-planar topography. For example, field oxide structures 110 and 120 in FIG. 1 cause an integrated circuit structure 100 to have a non-planar topography. During photolithographic processing of a polysilicon layer 130, light reflected from portions of polysilicon layer 130, near field oxide edges 111 and 121, cause a photoresist layer 140 to receive a non-uniform distribution of light energy. Consequently, the portions of photoresist layer 140 near field oxide edges 111 and 121 are overexposed, resulting in "notching" of polysilicon layer 130 near field oxide edges 111 and 121 in the final product. FIG. 2 shows a plan view of the polysilicon structure with notches 201-204 occurring in polysilicon layer 130 near edges of field oxide structures 110 and 120. Notching introduces inaccuracy in the desired polysilicon structure, which may degrade performance and reliability. The problems introduced by notching become more pronounced as the polysilicon structures get smaller with improvements in process technology. For example, notching is an acute problem for leading edge 0.35 micron (and beyond) process technology.
One solution to this problem is to cover the polysilicon layer with an anti-reflective coating (ARC) to reduce overexposure of the photoresist material. Amorphous silicon (a-Si), SiO.sub.x N, and TiN can be used to form an ARC. TiN is commonly used as an ARC because TiN has very good anti-reflective properties, and TiN deposition is well known. However, as shown in FIG. 3, during subsequent processing, Ti particles 310 from the TiN layer may diffuse through polysilicon layer 130 and contaminate layer 320 below polysilicon layer 130. The Ti particles 310 are able to diffuse through polysilicon layer 130 along the grain boundaries 330 in the polysilicon. The Ti contamination of layer 320 can cause serious problems in some integrated circuits. For example, if polysilicon layer 130 is used for forming a gate in a MOSFET, during subsequent processing, Ti particles 310 can diffuse through the polysilicon layer 130 to contaminate the gate oxide layer formed of SiO.sub.2, represented by layer 320. Under typical processing conditions, Ti will react with SiO.sub.2 in the gate oxide layer to form TiO.sub.2, thereby degrading the gate oxide layer's integrity and reliability. This would pose serious yield and reliability problems for leading edge MOSFET process technologies where gate oxides are getting progressively thinner.